Food consists mainly of water and the three main types of nutrients – protein, carbohydrate, and fat – that the body needs to survive. Before these nutrients can be used, their large molecules must be broken down in the digestive system into units small enough for the body to absorb. Absorption occurs in the small intestine (the duodenum, jejunum, and ileum) and the colon. Vitamins and minerals, which are also essential nutrients, consist of molecules tiny enough for the body to absorb without breaking them down first.
Food molecules are too large to pass through the walls of the digestive tract into the blood and lymph vessels and must be broken down into smaller molecules. Digestive enzymes help to break down proteins into amino acids, fats into fatty acids and glycerol, and starch (a complex carbohydrate) into simple sugars.
A particular enzyme combines with a large food molecule, such as a protein. The enzyme breaks down the large molecule into two or more smaller molecules.
The tiny molecules separate from the enzyme and are able to pass through the wall of the digestive tract into the bloodstream. The enzyme molecule remains unchanged.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.